The long term goal of this project is to develop an oral and non-radioactive breath test capable of determining the catalytic activity of CYP3A4 in patients. CYP3A4 appears to be the major single drug metabolizing enzyme present in human liver, and changes in the activity of this enzyme resulting from treatment with certain drugs appear to account in part some drug interactions. In the study proposed, the oral non-radioactive erythromycin breath test will be directly compared to the "gold standard" intravenously administered carbon-14 test in normal volunteers treated with either an inhibitor of CYP3A4 or an inducer of CYP3A4. The data obtained should determine whether the oral test can replace the intravenous test as a quantitiative measure of intrasubject variation in liver CYP3A4 activity. This would allow studies of liver CYP3A4 activity in children.